The service life of a bridge, pylon or the like is determined by the corrosion and fatigue of its steel. Extremely long service life is possible depending on corrosion resistance and fatigue resistance. Even in the case of current wear resistant steels, however, prevention of corrosion without a coating is difficult in regions near the seashore where salinity is high and regions where snow-melting salt is used. Painting, plating or other anti-corrosion treatment must be carried out regularly. In addition, joints, such as weld joints, experience metallic fatigue over long periods owing to the vibration produced by vehicles. The problem of having to carry out regular, large-scale repair work therefore arises.
FIG. 1 shows the results of atmosphere-exposure tests conducted on carbon steel and wear resistant steel in Japan. The data represent the results of atmosphere-exposure tests particularly at coastal industrial zones were corrosion is particularly great. Over a long test period of 10 years during which the SO.sub.x concentration increased, the amount of thickness loss, an index of amount of corrosion, reached 0.5 mm per side in the case of carbon steel. On the other hand, wear resistant steel turned in excellent results of under 0.2 mm. The need for this type of steel is therefore increasing more and more, and calls for further improvements have been heard.
Various solutions for these problems have been proposed. As typical examples, Japanese Unexamined Patent Publications No. 8(1996)-134587 and No. 9(1997)-165647 teach welded structural steels excellent in wear resistance that contain not greater than 0.15% of C and are further added with strengthening elements such as Mn, Ni and Mo to adjust to Ni+3Mo.gtoreq.1.2%, or Ni+Cu+3Mo.gtoreq.1.2%, Ceq: 0.5 or less. Japanese Unexamined Patent Publication No. 8(1986)-277439, teaches a steel composed of lath-like ferrite and cementite whose weld heat affected regions have high fatigue strength owing to a metallic texture containing as-transformed martensite at an area ratio of not less than 0.5% and not greater than 5%. Japanese Unexamined Patent Publication No. 9(1997)-249915 teaches elevation of the fatigue limit by adding appropriate amounts of Mn, Ti and B to make the texture bainite single phase without depending on cooling rate, while also achieving structural strengthening, utilizing precipitation of Cu and solid solution strengthening to enhance tensile strength and improve fatigue resistance, and also conducting rolling at a reduction ratio of not less than 30% in the unrecrystalized low-temperature zone or in the temperature range of the dual-phase region.
However, none of these conventional technologies enables endurance, without coating, against use in regions near the seashore where salinity is high and regions where snow-melting salt is used. As in the past, joints, such as weld joints, experience metallic fatigue over long periods owing to the vibration produced by vehicles, so that regular, large-scale repair work is necessary.